1. Field of the Invention
The present invention relates to a discharge device for pulsed laser which performs pulsed laser oscillation by preionizing across main discharge electrodes disposed in a laser medium and performing a main discharge to excite the laser medium.
2. Description of the Prior Art
TEA laser causes a uniform glow discharge across a pair of opposing main discharge electrodes to form an inverted population region necessary for laser oscillation.
To obtain the glow discharge spread to fill the entire main discharge gap, the TEA laser has to cause preionization before starting the main discharge to ionize the entire main discharge gap previously. Especially, an excimer laser has to ionize as many as possible immediately before the main discharge because the electrons in a negative gas have a short lifetime.
Currently, various types of methods using X rays, a spark discharge, a corona discharge or the like are used as a preionizing method. Among them, a method using the corona discharge is extensively used because it is relatively simple and easy and has less contamination of a laser gas.
FIG. 8 shows an equivalent circuit of a conventional capacity shift type magnetic pulse compression discharge device, namely of a discharge device for pulsed laser, which employs the corona discharge for the preionization. FIG. 9 shows an example of voltage and electric current waveforms at respective points of the discharge device for pulsed laser shown in FIG. 8.
In the discharge device for pulsed laser shown in FIG. 8, corona preionization capacitor (hereinafter called the preionization capacitor) Cpp and corona preionization electrode (hereinafter called the preionization electrode) 4 are disposed to preionize in main discharge gap 3 which is formed across a pair of main discharge electrodes 1, 2, and a laser medium in the main discharge gap 3 across the main discharge electrodes 1, 2 is preionized by UV (ultraviolet) light produced by a corona discharge at the preionization electrode 4.
The discharge device for pulsed laser shown in FIG. 8 has two-stage magnetic pulse compression circuit utilizing a saturation phenomenon of three magnetic switches AL0 to AL2 each made of a saturable reactor.
In the discharge device for pulsed laser shown in FIG. 8, an electric charge is applied from high-voltage power source HV to capacitor C0 through the magnetic switch AL0 and coil L1.
Then, when pulse oscillation synchronizing signal (trigger signal) TR, which is turned on in synchronization with a repetition frequency of a pulsed laser oscillation, is input, main switch SW is turned on (at time t0 shown in FIG. 9). When the main switch SW is turned on, electric potential VSW of the main switch SW drops sharply to zero. When time integral (namely, time integral value of voltage VC0) S0 of voltage difference xe2x80x9cVC0xe2x88x92VSWxe2x80x9d between the capacitor C0 and the main switch SW as voltages at both ends of the magnetic switch AL0 reaches a threshold value which is based on a set characteristic of the magnetic switch AL0, the magnetic switch AL0 is saturated at time tl, and electric current pulse i0 flows through a loop of the capacitor C0, the magnetic switch AL0, the main switch SW and capacitor C1.
Duration xcfx840 in which the electric current pulse i0 starts to flow and becomes 0 (time t2 shown in FIG. 9), namely electric charge transfer time xcfx840 in which the electric charge is completely transferred from the capacitor C0 to the capacitor C1, is based on respective capacitance of the inductance, the capacitor C0 and the capacitor C1 after the magnetic switch AL0 is saturated with loses due to the main switch SW and the like disregarded.
Meanwhile, when time product S1 of the voltage VC1 of the capacitor C1 reaches the threshold value which depends on the set characteristic of the magnetic switch AL1, the magnetic switch AL1 is saturated and has a low inductance at time t3. Thus, electric current pulse i1 flows through a loop of the capacitor C1, capacitor C2 and the magnetic switch AL1. The electric current pulse i1 becomes 0 at time t4 after a lapse of predetermined transfer time xcfx841 which is determined by an inductance after the saturation of the magnetic switch AL1 and the capacitance of the capacitors C1, C2.
When time product S2 of voltage VC2 of the capacitor C2 reaches a threshold value which is based on a set characteristic of the magnetic switch AL2, the magnetic switch AL2 is saturated at time t5. Thus, electric current pulse i2 flows through a loop of the capacitor C2, peaking capacitor CP and the magnetic switch AL2. The flow of the electric current pulse i2 rises voltage Vcp of the peaking capacitor Cp and voltage VCpp of the preionization capacitor Cpp.
Then, when the voltage of the preionization electrode 4 rises to a predetermined preionization start voltage through the preionization capacitor Cpp, a corona discharge is caused at the preionization electrode 4 to flow electric current i3, and the main discharge gap 3 is preionized.
Besides, the voltage VCp of the peaking capacitor Cp rises further with the progress of charging. And, when the voltage VCp reaches a predetermined main discharge start voltage, a laser gas between the main discharge electrodes 1, 2 is undergone an electrical breakdown at time t6, and a main discharge is started across the main discharge electrodes 1, 2 to flow electric current i4. The laser medium is excited by the main discharge caused across the main discharge electrodes 1, 2, and laser light is emitted in several nsec.
Then, the voltages of the peaking capacitor Cp and the preionization capacitor Cpp drop sharply owing to the main discharge and return to the states before the charging was started after a lapse of a predetermined period.
Such a discharge operation is repeated by the switching operation of the main switch SW which is synchronized with the trigger signal TR to perform pulsed laser oscillation at a predetermined repetition frequency (namely, a pulse oscillation frequency).
In such a case, because it is determined that the electric charge transfer circuit of each stage which is comprised of the magnetic switch and the capacitor has smaller inductance toward later stages, the pulse compression operation is performed so that the peak values of the electric current pulses i0 to i2 become high sequentially and the electrifying duration becomes shorter sequentially. As a result, an intense discharge is caused across the main discharge electrodes 1, 2 in a short period of time.
In the magnetic pulse compression circuit, duration td (hereinafter called the emission delay time) from the time t0 when the trigger signal TR is input and the main switch SW is turned on to the time t6 when the laser light is actually emitted depends on electrifying widths xcfx840, xcfx841, xcfx842 of the electric current pulses i0, 01, i2 and saturation time "sgr"0, (xcfx840+"sgr"1), (xcfx841+"sgr"2) of the respective magnetic switches AL0 to AL2.
The electrifying widths (electric charge transfer time) xcfx840, xcfx841, xcfx842 are based on the inductance of the magnetic switch and the capacitance of the capacitor included in the electric charge transfer circuits of the respective stage, and such values are greatly influenced by the atmosphere temperature of the magnetic pulse compression circuit.
Besides, variations "sgr"0, "sgr"1, "sgr"2 in the saturation time are based on a time integral of the voltage applied to the respective magnetic switches AL0 to AL2, so that they are greatly influenced by the voltage V0 of the high-voltage power source HV.
A discharge device other than the discharge device for pulsed laser shown in FIG. 8 is disclosed in Japanese Patent Application No. 9-271207 (Laid-Open Publication No. 11-112300) filed in Japan in the name of the applicant of this patent application.
Density Ne0 of electrons produced by the preionization and its spatial distribution have an influence upon the growth and stability of a glow discharge in the high-pressure laser gas. And, they are included in the factors which exert an influence upon the laser output power, the spatial intensity distribution of the laser beam, the pulse width and the like.
With the increase of the electron density Ne0, the generation and maintenance of the stable glow discharge are facilitated, and the laser output power obtained is also increased.
But, in the above conventional discharge device for pulsed laser, the main discharge (namely, the glow discharge) by the main discharge electrodes 1, 2 is started in a state that the preionization in the main discharge gap by the corona discharge is not performed sufficiently, namely in a state before reaching the aforesaid electron density Ne0 with that the stable glow discharge can be caused and maintained. Therefore, the stability and oscillation efficiency of the energy of the pulsed laser light subject to a pulse oscillation are degraded.
In other words, the preionization capacitor Cpp is connected to the peaking capacitor Cp, so that the preionization capacitor Cpp is also charged in synchronization with the charging to the peaking capacitor Cp.
Therefore, emission timing of the corona discharge (preionization discharge) by the preionization electrode 4 depends on the voltage applied to the main discharge electrodes 1, 2, and the duration between the corona discharge by the preionization electrode 4 and the main discharge by the main discharge electrodes 1, 2 becomes short.
Accordingly, the main discharge (namely, the glow discharge) by the main discharge electrodes 1, 2 is started before the aforesaid electron density Ne0 in the main discharge gap 3 reaches an electron density with that the stable glow discharge can be caused and maintained.
Especially, in view of the energy stability and the luminous efficiency, the pulse width of the voltage applied to the main discharge electrode tends to become short by the high-repetitive oscillation of the excimer laser, and the emission timing of the preionization also tends to be close to the main discharge start time.
Therefore, the emission timing of the actual preionization might deviate from the emission timing of the optimum preionization, and the energy stability and the luminous efficiency might be degraded.
As described above, the conventional discharge device for pulsed laser has a short duration between the corona discharge by the preionization electrode 4 and the main discharge by the main discharge electrodes 1, 2. Therefore, the main discharge is caused with the preionization in an insufficient state, and the energy stability of the pulsed laser light and the high oscillation efficiency cannot be obtained.
The present invention has been achieved in view of the circumstances described above, and it is an object of the invention to provide a discharge device for pulsed laser which has the main discharge by main discharge electrodes and the preionization discharge by a preionization electrode realized by a separate discharge system and can set an emission timing of the preionization by the preionization electrode arbitrarily.
Accordingly, a first aspect of the present invention is directed to a discharge device for pulsed laser, which comprises main discharge means having main discharge electrodes disposed in a laser medium, a main discharge capacitor connected in parallel with the main discharge electrodes, a magnetic pulse compression circuit which is connected in parallel with the main discharge capacitor and has a saturable reactor and a forwarding capacitor connected in series, and a power source for supplying electric charges to the forwarding capacitor, and which generates pulsed laser by transferring the electric charges accumulated in the forwarding capacitor according to a preset pulse oscillation synchronizing signal to the main discharge capacitor by utilizing a magnetic saturation phenomenon of the saturable reactor to cause a pulse discharge across the main discharge electrodes so to excite the laser medium, wherein:
preionization means for causing preionization across the main discharge electrodes is disposed to discharge independent of the main discharge means; and
preionization is caused across the main discharge means by the preionization means before the main discharge is caused by the main discharge means.
According to the first aspect of the invention, the equivalent circuit of the main discharge circuit section (corresponding to the main discharge means) has high-voltage power source HV, main switch SW, four capacitors C0, C1, C2, Cp and main discharge electrodes 1, 2 as shown in FIG. 1. And, there are formed a first stage electric charge transfer circuit which forms electric current loop i0 from the capacitor C0 to the capacitor 1 through magnetic switch AL0 and the main switch SW, a second stage electric charge transfer circuit which forms electric current loop i1 from the capacitor C1 to the capacitor C2 through magnetic switch AL1, and a third stage electric charge transfer circuit which forms electric current loop i2 from the capacitor C2 to the peaking capacitor (namely, main discharge capacitor) Cp through magnetic switch AL2.
This equivalent circuit has a two-stage magnetic pulse compression circuit utilizing a saturation phenomenon of the three magnetic switches AL0 to AL2 made of the saturable reactor. And, the capacitors C1, C2 function as the aforesaid forwarding capacitor. Besides, the magnetic switches AL1, AL2 function as the aforesaid saturable reactor.
The main discharge circuit section and the preionization discharge circuit (corresponding to the above preionization discharge means) 10 are designed to have a discharge system formed of an independent circuit structure.
The preionization discharge circuit 10 causes preionization of the main discharge gap 3 at an arbitrary determined moment when the main discharge by the above main discharge circuit section is not caused yet (FIG. 5).
Therefore, according to the first aspect of the invention, the emission timing of the preionization discharge (namely, the corona discharge) is not limited to that of the main discharge but can be determined arbitrarily.
A second aspect of the invention is directed to the discharge device for pulsed laser according to claim 1, wherein the preionization means comprises:
A preionization electrode for preionizing;
induction type energy accumulation means for accumulating energy to cause a preionization discharge by the preionization electrode;
supply means for supplying energy to the energy accumulation means; and
switching means which performs a switching operation in correspondence with the pulse oscillation synchronizing signal to supply energy from the supply means to the energy accumulation means and to supply the energy accumulated in the energy accumulation means to the preionization electrode.
According to the second aspect of the invention, in the equivalent circuit of the preionization discharge circuit 10, preionization electrode 11 for preionizing the main discharge gap 3, coil L (corresponding to the aforesaid induction type energy accumulation means) 12 as an induction type energy accumulation circuit, and constant current source (corresponding to the supply means) 13 are connected in series as shown in FIG. 1. And, switch SW (corresponding to the switching means) 14, which is a switching element such as an insulated gate bipolar transistor (IGBT), a gate turn-off thyrister (GTO thyrister) or a thyratron to make a switching operation according to the predetermined timing, is connected in parallel to the preionization electrode 11.
According to the second aspect of the invention, the coil L12 can increase a voltage rising rate (dV/dt) as compared with the case of the preionization capacitor Cpp (see FIG. 8) used for the conventional discharge device for pulsed laser. Therefore, a corona emission intensity to the same input energy can be increased, and additional preionization can be performed in the main discharge gap 3.
A third aspect of the invention relates to the first or second aspects of the invention which further comprises predicting means for predicting a start moment of a main discharge by the main discharge electrodes; and control means for controlling the preionization means so that preionization is caused across the main discharge electrodes earlier by a preset time than the start moment of the main discharge predicted by the predicting means.
A fourth aspect of the invention relates to the second aspect of the invention which further comprises:
predicting means for predicting a start moment of the main discharge by the main discharge electrodes; and
control means for controlling the switching means so that preionization is caused across the main discharge electrodes earlier by a preset time than the start moment of the main discharge predicted by the predicting means.
According to the third aspect of the invention, preionization discharge control section (corresponding to the aforesaid prediction means and control means) 40 predicts a start moment (laser emission moment) of the main discharge by the aforesaid main discharge circuit section, and particularly by the main discharge electrodes 1, 2, according to pulse oscillation synchronizing signal TRL output from laser controller 20 and delay time xcex4xe2x80x2 and outputs a signal indicating the predicted laser emission moment, namely preionization discharge timing signal Ydt, to the preionization discharge circuit section 10.
The preionization discharge control section 40 has a memory table in which data indicating the standard delay time Tds is stored, and recognizes that a value obtained by subtracting the delay time xcex4xe2x80x2 from the standard delay time Tds is actual emission delay time td and that a value obtained by adding the actual emission delay time td from a rising moment of the pulse oscillation synchronizing signal TRL is an emission moment of the laser light.
And, when the preionization discharge control section 40 recognizes that the laser emission is performed after a lapse of the actual emission delay time td (=standard delay time Tdsxe2x88x92delay time xcex4xe2x80x2) as shown in FIGS. 5(b), (c) from the rising moment of the pulse oscillation synchronizing signal TRL as shown in FIG. 5, (a), the control section 40 outputs the preionization discharge timing signal Ydt to the main switch SW14 so to cause preionization discharge (corona discharge) earlier by predetermined time tyd than the moment of the laser emission as shown in FIGS. 5, (d), (e).
According to the third and fourth aspects of the invention, the emission timing of the preionization discharge (namely, the corona discharge) can be determined arbitrarily without being restricted by the emission timing of the main discharge. Thus, the emission of the corona discharge can be made when the main discharge is caused after the preionization is sufficiently caused in the main discharge gap.
A fifth aspect of the invention relates to the discharge device for pulsed laser according to the fourth aspect of the invention, wherein the predicting means comprises:
voltage compensating means for compensating variations in an actual emission moment of the pulsed laser due to a change in a power-supply voltage depending on a voltage instruction value to the power source;
temperature compensating means for compensating variations in the actual emission moment of the pulsed laser due to a change in an atmosphere temperature in the magnetic pulse compression circuit; and
actual emission moment predicting means for predicting the actual emission moment of the pulsed laser according to the actual emission moment compensated by the voltage compensating means and the actual emission moment compensated by the temperature compensating means; and wherein
the predicting means recognizes the actual emission moment predicted by the actual emission moment predicting means as a start moment of the main discharge.
According to the fifth aspect of the invention, laser controller 20, temperature sensor 30 and preionization discharge control section 40 provide the function of the aforesaid predicting means, and the preionization discharge control section 40 also has the function of the above control means. Besides, voltage instruction value arithmetic section 21, standard delay time setting section 22 and delay time arithmetic section 23 provide the function of the aforesaid voltage compensation means, and the temperature sensor 30 and temperature compensation section 24 provide the function of the above temperature compensation means. Delay section 25 produces pulse oscillation synchronizing signal TRL with compensation of variations in actual emission moment of the pulsed laser due to a change in power-supply voltage corresponding to the voltage instruction value and a change in atmosphere of the magnetic pulse compression circuit.
The preionization discharge control section (corresponding to the actual emission moment predicting means of the predicting means) 40 recognizes that the laser emission is performed after a lapse of the above actual emission delay time td (=standard delay time Tdsxe2x88x92delay time xcex4xe2x80x2) as shown in FIGS. 5(b), (c) from the rising moment of the pulse oscillation synchronizing signal TRL as shown in FIG. 5, (a).
According to the fifth aspect of the invention, the actual emission moment of the laser light can be predicted considering variations in actual emission moment of the pulsed laser due to a change in power-supply voltage according to the voltage instruction value and a change in atmosphere of the magnetic pulse compression circuit, and the preionization discharge can be caused earlier by a predetermined period than the predicted actual emission moment.
A sixth aspect of the invention relates to the discharge device for pulsed laser according to the second aspect of the invention, wherein:
the preionization means has the switching means, the preionization electrode, the energy accumulation means and the supply means connected in series; and
one end of the preionization electrode is connected to the switching means and the other end of the preionization electrode is connected to the energy accumulation means.
According to the sixth aspect of the invention, preionization electrode (corresponding to the above preionization electrode) 11 is comprised of electrode 11A which is a conductor (e.g., metal), cylindrical tube 11B as a dielectric through which the electrode 11A is inserted, and continuity plate 11C which is disposed to face the tube 11B so to enable preionization of the main discharge gap 3 (see FIG. 1) across the main discharge electrodes 1, 2 and is grounded as shown in FIG. 2.
In the preionization discharge circuit (corresponding to the above preionization means) 10 having the preionization electrode 11, constant current source 13 is connected to one end of coil L12, the other end of the coil L12 is connected to one end of the electrode 11A of the preionization electrode 11, and the other end of the electrode 11A is connected to switch SW14.
According to the sixth aspect of the invention, the electrode 11A can also be used as a part of the induction type energy accumulation means, and when the switch SW14 is changed from on to off, a uniform electric field is produced in the electrode 11A in its longitudinal direction. Therefore, the corona discharge can be obtained with a uniform electric field intensity at the preionization electrode 11 in its longitudinal direction.